1. Field of the Invention
This specification relates to reinforced hollow fiber membranes, to reinforcement structures for hollow fiber membranes, and to methods of making hollow fiber membranes and reinforcement structures for them.
2. Description of the Related Art
The following is not an admission that anything discussed below is citable as prior art or part of the common general knowledge.
Polymeric separation membranes in the form of small capillary tubes or hollow fibres can be made from a variety of polymers by different methods that can be categorized as NIPS (non-solvent induced phase separation), or TIPS (thermally induced phase separation). The membranes may have a separation layer on the outside and may be used, for example, for microfiltration (MF) or ultrafiltration (UF).
The simplest approach is to make the hollow fiber from a single material that provides mechanical properties (tensile resistance, burst or collapse resistance, and resistance to fatigue by cracking), separation properties (rejection and permeability) and chemical properties (tolerance to chemicals in the feed or to cleaners). In many instances where it is not possible to find the right compromise, composite membranes are prepared. In a composite membrane a microporous support structure is coated with the separation layer.
A special case of composite hollow fibre membrane is that where a hollow textile braided sleeve is coated or impregnated with a polymeric membrane. The braid provides the strength and flexibility that is needed in MF/UF applications such as filtration of water suspensions where continuous or intermittent agitation (with air or otherwise) of the hollow fibers is needed to prevent fouling or accumulation of solids on the membrane surface.
There are several examples of braid-supported filtration membranes in the prior art. These include, U.S. Pat. No. 4,061,861 to Hayano et al. where a polymer is impregnated into a hollow braid to prevent shrinkage when operating at high temperature; U.S. Pat. No. 5,472,607 and U.S. Pat. No. 6,354,444 to Mahendran et al.; U.S. Pat. No. 7,267,872 to Lee et al. where the membrane is coated on the outside surface of the braid and penetration is limited; and, U.S. Pat. No. 7,306,105 to Shinada et al. where the braid is coated with 2 different porous layers.
Braid-supported hollow fibre membranes are normally prepared as follows. The braid is fabricated on a braider, wound on a bobbin, and transferred to a spin line where it is coated or impregnated with a polymer solution. Relatively thick wall braids are used in order for the final hollow fibre membrane to be round in shape. This is necessary because the braid must be round-stable, i.e., prevented from flattening out through winding and unwinding and must be round when inserted in the coating spinneret.
Round-stable braids are thus normally made with a thick wall and have two major disadvantages. First, the ratio of inside-to-outside diameters is small, typically smaller than 0.6. The ratio of inside-to-outside diameters is the normalized parameter that determines the pressure loss to conduct permeate through the lumen of the membrane. The high lumen pressure drop in thick wall braids thus limits the length of hollow fibres that can be potted in a module. Second, round-stable braids are fabricated with a large number of braiding carriers. Each carrier is supplied from a different bobbin and the bobbins must cross paths in the braiding machine. The bobbins must start from a stop position, accelerate and decelerate and come back to a stop position every time the carriers cross each other. This is a very slow operation. Small diameter braids (less than 2 mm) are normally made at a speed of less than 0.5 m/min. In contrast, the braid coating/impregnation operation is typically done much faster, at a speed of greater than 15 m/min.